Steel excellent in corrosion resistance and processability

ABSTRACT

This invention relates to a steel excellent in corrosion resistance and processability, and more particularly to a steel excellent in corrosion resistance for the exhaust system of an internal combustion engine of, for example, an automobile or ship. Such steel contains not less than 0.01% and less than 1.2% of Si, 0.1-1.5% of Mn, 5.5-9.9% of Cr and 0.3-3.0% of Al, is reduced to a content of C of not more than 0.02%, P of not more than 0.03%, S of not more than 0.01% and N of not more than 0.02%, contains Nb, V, Ti, Zr, Ta and Hf in a total amount of 0.01-0.5%, satisfies the relationship set out below, and optionally contains in addition one or more of Cu, Mo, Sb, Ni, W, rare earth elements, and Ca, the remainder being Fe and unavoidable impurities. 
     
         (Nb/93)+(V/51)+(Ti/48)+(Zr/91)+(Ta/181)+(Hf/179)-0.8×[(C/12)+(N/14)]. 
    
     gtoreq.0

TECHNICAL FIELD

This invention relates to a steel that is excellent in corrosionresistance and processability and more particularly to a steel thatexhibits excellent corrosion resistance in the exhaust system of aninternal combustion engine such as of an automobile, ship or the likeand which also exhibits excellent processability during processing intocomponents.

BACKGROUND ART

In the exhaust systems of internal combustion engines, particularlythose used in automobiles, corrosion from the internal and externalsurfaces has conventionally been suppressed by the use of ordinary steelplated with aluminum or zinc. However, since exhaust systems are nowequipped with catalytic converters or the like for purifying the exhaustgas to reduce environmental pollution, the corrosion resistance providedby such plated steels has become inadequate. As taught, for example, byJapanese Patent Public Disclosures No. Sho 63-143240 and 63-143241,therefore, steels containing 5-10% of Cr were developed for increasingthe corrosion resistance of the steel base material. In response torecent increases in the service life and warranty period of automobiles,however, large amounts of high-grade stainless steels containing around18% of Cr, with or without addition of Mo, are now being used in exhaustsystems. Even such high-grade stainless steels have been found to beinsufficient in corrosion resistance and subject to localized corrosion(pitting) and the like. Moreover, since the large amounts of Cr and Mocontained in such high-grade stainless steels degrades theirprocessability, they are extremely difficult to fabricate into complexlyshaped components such as those used in exhaust systems, and the highlycomplicated manufacturing processes required increases the processingcost. They may even be completely unusable for some shapes and, at anyrate, are expensive as a raw material.

When a steel with a given Cr content becomes more susceptible to pittingowing to a more severe operating environment, as is typical in exhaustsystems, the problem is generally coped with by further increasing theCr or Mo content so as to strengthen the resistance to corrosion.

In light of these circumstances, the object of the present invention isto provide a steel which exhibits strong resistance to a corrosiveenvironment such as present in the exhaust system of an internalcombustion engine, exhibits excellent processability, and is low incost.

DISCLOSURE OF THE INVENTION

For achieving this object, the inventors conducted studies from variouspoints of view in order to develop a steel with excellent corrosionresistance in corrosive environments, particularly in the corrosiveenvironment of an exhaust system. Beginning their studies with thecorrosive environment of exhaust systems, they discovered that corrosionof an internal combustion engine exhaust system occurs in an environmentof chloride ions, sulfate ions and the like heated to 80°-150° C. Theynext examined various means for increasing corrosion resistance in sucha corrosive environment and discovered that, exactly opposite from inprior art stainless steels, a steel with a reduced Cr content of5.5-9.9% and added with 1.0-3.0% of Al exhibits extraordinarily goodcorrosion resistance in exhaust systems and other such corrosiveenvironments. Additional studies directed to providing a furtherimproved steel revealed that when the foregoing steel is reduced in Cand N content and added with Nb, V, Ti, Zr, Ta and Hf to meet aprescribed condition, its corrosion resistance is increased and itsprocessability improved, that when the foregoing steel is added with Cu,Mo, Sb, Ni and W, independently or in combination, its corrosionresistance is improved, and that Si and Mn are suitable deoxidation andstrengthening elements for the steel.

The present invention was accomplished on the basis of the foregoingknowledge. In its first aspect, the invention resides in a steelexcellent in corrosion resistance and processability which, in weightper cent, contains not less than 0.01% and less than 1.2% of Si,0.1-1.5% of Mn, 5.5-9.9% of Cr and 1.0-3.0% of Al, is reduced to acontent of C of not more than 0.02%, P of not more than 0.03%, S of notmore than 0.01% and N of not more than 0.02%, contains one or moreelements selected from among Nb, V, Ti, Zr, Ta and Hf in a total amountof 0.01-0.5%, and satisfies the relationship

    (Nb/93)+(V/51)+(Ti/48)+(Zr/91)+(Ta/181)+(Hf/179)-0.8×[(C/12)+(N/14)].gtoreq.0,

the remainder being Fe and unavoidable impurities.

In its second aspect, the invention resides in a steel which as additioncomponents to those of the steel according to the first aspect of theinvention contains, in weight per cent, one or more of 0.05-3.0% of Cu,0.05-2.0% of Mo, 0.01-0.5% of Sb, 0.01-2.0% of Ni and 0.05-3.0% of W.

In its third aspect, the invention resides in a steel which as additioncomponents to those of the steel according to the first aspect of theinvention contains, in weight per cent, one or more of 0.001-0.1% ofrare earth elements and 0.0005-0.03% of Ca.

In its fourth aspect, the invention resides in a steel which as additioncomponents to those of the steel according to the second aspect of theinvention contains, in weight per cent, one or more of 0.001-0.1% ofrare earth elements and 0.0005-0.03% of Ca.

BEST MODE FOR CARRYING OUT THE INVENTION

The reason for the limitation of the content ranges of the individualComponents in the invention will be described in the following.

Si: In a steel containing 5.5% or more of Cr, added Si is effective as adeoxidizing agent and strengthening element. However, it does notmanifest a sufficient deoxidizing effect when present at a content ofless than 0.01% and, when present at 1.2% or more, not only experiencesa saturation of effect but also degrades processability. The contentrange thereof is therefore limited to not less than 0.01% and less than1.2%.

Mn: Mn is necessary as a deoxidizing agent for the steel and is requiredto be present at a content of not less than 0.1%. When contained inexcess of 1.5%, however, not only does Mn experience a saturation ofeffect but the excessive Mn content degrades processability. Its uppercontent limit is therefore set at 1.5%.

Cr: Cr must be added to a content of at least 5.5% for securingcorrosion resistance but when added to a content exceeding 9.9% not onlyunnecessarily increases cost but also degrades processability. Its uppercontent limit is therefore set at 9.9%.

Al: Like Cr, Al is an important element in this invention for securingcorrosion resistance. As pointed out earlier, Al does not sufficientlysuppress pitting at a content of less than 1.0% and, on the other hand,experiences a saturation of effect and degrades processability whenadded in excess of 3.0%. The content range of Al is therefore limited to1.0-3.0%.

C, N: C and N degrade the processability of the steel sheet and, inaddition, C degrades corrosion resistance by forming a carbide with Crwhile N degrades toughness. Since low contents of C and N are thereforedesirable, the upper content limit of both is set at 0.02% and thelowest possible content of both is preferable.

P: A low content of P is preferable because it degrades toughness whenpresent in a large amount. The upper content limit thereof is thereforeset at 0.03%.

S: A low content of S is preferable because it degrades pittingresistance when present in a large amount. The upper content limitthereof therefore is set at 0.01%.

Nb, V, Ti, Zr, Ta, Hf: By fixing the C and N in a high Cr steel ascarbides and nitrides, Nb, V, Ti, Zr, Ta and Hf manifest a pronouncedeffect toward increasing corrosion resistance and improvingprocessability. Although they can be added individually or incombinations of two or more, they have no effect when added eitherindividually or in combination to a total amount of less than 0.01%,while when added in excess of 0.5% they not only unnecessarily increasescost but also become a cause of roll marks. The upper content limit istherefore set at 0.5%. For effectively improving processability,moreover, the amount of added Nb, V, Ti, Zr, Ta, and Hf must satisfy therelationship

    (Nb/93)+(V/51)+(Ti/48)+(Zr/91)+(Ta/181)+(Hf/179)-0.8×[(C/12)+(N/14)].gtoreq.0.

While the basic components of the steel excellent in corrosionresistance and processability aimed at by the present invention are asset out in the foregoing, the invention is also directed to a steelwhose properties are further improved by additionally containing asrequired, the elements set out below.

Cu: When added at not less than 0.05% to a steel containing not lessthan 5.5% of Cr and not less than 1.0% of Al, Cu has an effect ofincreasing resistance to total surface corrosion. When added in excessof 3.0%, however, it not only experiences a saturation of effect butalso degrades hot workability. Its upper content limit is therefore setat 3.0%.

Mo: When added at not less than 0.05% to a steel containing not lessthan 5.5% of Cr and not less than 0.3% of Al, Mo has an effect ofsuppressing the occurrence and growth of pits. When added in excess of2.0%, however, it not only experiences a saturation of effect but alsodegrades processability. Its upper content limit is therefore set at2.0%.

Sb: When added at not less than 0.01% to a steel containing not lessthan 5.5% of Cr and not less than 1.0% of Al, Sb has an effect ofimproving resistance to pitting and total surface corrosion. When addedin excess of 0.5%, however, it degrades workability. Its upper contentlimit is therefore set at 0.5%.

Ni: When added at not less than 0.01% to a steel containing not lessthan 5.5% of Cr and not less than 1.0% of Al, Ni has an effect ofsuppressing pitting. When added in excess of 2.0%, however, it not onlyexperiences a saturation of effect but also degrades hot workability.Its upper content limit is therefore set at 2.0%.

W: When added at not less than 0.05% to a steel containing not less than5.5% of Cr and not less than 1.0% of Al, W has a pronounced effect ofsuppressing the occurrence and growth of pits. When added in excess of3.0%, however, it not only experiences a saturation of effect but alsodegrades processability. Its upper content limit is therefore set at3.0%.

Rare earth elements (REM), Ca: Rare earth elements and Ca are elementshaving an effect of increasing hot workability and improving pittingresistance. Sufficient effect is not manifested at an addition amount ofrare earth elements of less than 0.001% or of Ca of less than 0.0005%,while addition of rare earth elements in excess of 0.1% or of Ca inexcess of 0.03% has the adverse effects of degrading hot workability,owing to the formation of coarse nonmetallic inclusions, and degradingpitting resistance. The upper content limit of rare earth elements istherefore set at 0.1% and that of Ca at 0.03%. In this invention "rareearth elements" means the elements with atomic numbers 57-71 and 89-103and Y.

When the steel proposed by the present invention is used in the exhaustsystem of an internal combustion engine, it is first produced as a steelsheet which can thereafter be formed into a prescribe shape using apress or the like and then fabricated into a product by processing andwelding. Alternatively, the steel sheet can be first formed into a tubesuch as an electric welded steel tube and subject to secondaryprocessing and welding for use as the product. All steels having thecomposition and combination of elements defined by the presentinvention, whether processed by these or other processes, are subjectsof the invention. The optimum production processes can be selected inlight of cost, limitations of existing production facilities and thelike, and no selection of a process results in a deviation from theinvention. In addition, the steel proposed by the invention can beapplied not only to internal combustion engine exhaust systems but alsoto various other corrosive environments such as environments in which itis exposed to high-temperature aqueous solutions containing chlorideions, sulfate ions and the like or in which heating and cooling occurrepeatedly.

EXAMPLES

Examples of the invention will now be explained.

Steels of the compositions shown in Tables 1-8 were formed into steelsheets of a thickness of 1 mm by ordinary steel sheet productionprocesses including melting, hot rolling, cold rolling and the like, andwere then annealed at 850° C. A test specimen measuring 50 mm in widthand 70 mm in length was cut from each sheet and subjected to a corrosiontest. The test was carried out by repeating twenty times the process ofimmersing one half of the specimen in 50 cm³ of an aqueous solutionadded with 100 ppm of sulfate ions, 100 ppm of chloride ions and 500 ppmof bicarbonate ions in the form of ammonium salt, maintaining theatmosphere of the vessel at 130° C. and completely evaporating andvolatilizing the test solution. This was for simulating the corrosivecondition of an automobile exhaust system. The results of the corrosiontest are shown in Tables 2, 4, 6 and 8. In the corrosion test results ofTables 2, 4, 6 and 8, ⊚ indicates a maximum corrosion depth of not morethan 0.15 mm, O a maximum corrosion depth of not more than 0.2 mm, and Xa maximum corrosion depth exceeding 0.2 mm. Processability was evaluatedby conducting a cup test at a reduction ratio of 1.8 and checking foroccurrence of cracking. These test results are also shown in Tables 2,4, 6 and 8. O indicates good results in the cup test and X indicatesthat cracking occurred in the cup test.

As is clear from Tables 1-8, the invention steels Nos. 1-17, 19-37 andNos. 50, 52-57, 59, 61, 65, 67, 69, 71, 72, 75 and 77-86 exhibited goodcorrosion resistance even in an extremely harsh corrosive environment(exhaust environment) and were also excellent in processability, whilethe comparison steels Nos. 38-49 and Nos. 87-98 were inferior in bothcorrosion resistance and processability.

                                      TABLE 1                                     __________________________________________________________________________    Composition (Wt %)                                                            No.                                                                              C  Si Mn P  S  Al Cr                                                                              Nb V  Ti Zr Ta                                         __________________________________________________________________________    INVENTION STEELS                                                              1  0.006                                                                            0.90                                                                             0.25                                                                             0.026                                                                            0.003                                                                            1.47                                                                             5.8                                                                             0.10                                                   2  0.007                                                                            0.47                                                                             1.02                                                                             0.026                                                                            0.005                                                                            1.53                                                                             6.8  0.05                                                3  0.004                                                                            0.63                                                                             1.14                                                                             0.027                                                                            0.006                                                                            1.62                                                                             9.8     0.035                                            4  0.008                                                                            0.24                                                                             0.58                                                                             0.022                                                                            0.009                                                                            2.56                                                                             5.7        0.14                                          5  0.005                                                                            0.11                                                                             0.80                                                                             0.022                                                                            0.006                                                                            2.91                                                                             9.2           0.16                                       6  0.003                                                                            0.19                                                                             0.50                                                                             0.022                                                                            0.008                                                                            2.13                                                                             8.9                                                      7  0.006                                                                            0.52                                                                             0.45                                                                             0.021                                                                            0.006                                                                            1.55                                                                             8.1     0.104                                            8  0.005                                                                            0.53                                                                             0.71                                                                             0.027                                                                            0.006                                                                            1.39                                                                             8.0                                                                             0.08                                                   9  0.005                                                                            0.69                                                                             1.10                                                                             0.021                                                                            0.004                                                                            1.41                                                                             7.8  0.04                                                10 0.009                                                                            0.32                                                                             0.68                                                                             0.022                                                                            0.004                                                                            1.44                                                                             7.5        0.12                                          11 0.005                                                                            0.95                                                                             0.80                                                                             0.024                                                                            0.007                                                                            1.42                                                                             8.1  0.01     0.15                                       12 0.006                                                                            0.12                                                                             0.77                                                                             0.020                                                                            0.004                                                                            1.61                                                                             7.7                                                                             0.08  0.107                                                                            1                                             13 0.005                                                                            0.45                                                                             0.61                                                                             0.026                                                                            0.005                                                                            1.22                                                                             7.4     0.042                                                                            0.01                                          14 0.007                                                                            0.71                                                                             0.60                                                                             0.019                                                                            0.006                                                                            1.49                                                                             9.1                                                      15 0.005                                                                            0.74                                                                             0.63                                                                             0.023                                                                            0.005                                                                            1.68                                                                             9.3                                                                             0.12                                                   16 0.006                                                                            1.05                                                                             0.47                                                                             0.025                                                                            0.004                                                                            1.48                                                                             8.3  0.11                                                17 0.006                                                                            0.76                                                                             0.88                                                                             0.019                                                                            0.008                                                                            1.51                                                                             8.2  0.05                                                19 0.003                                                                            0.73                                                                             1.07                                                                             0.024                                                                            0.007                                                                            2.11                                                                             9.1        0.07                                          20 0.009                                                                            0.58                                                                             1.12                                                                             0.026                                                                            0.005                                                                            1.84                                                                             8.5                                                                             0.32                                                                             0.03                                                21 0.011                                                                            0.36                                                                             0.80                                                                             0.025                                                                            0.005                                                                            2.69                                                                             6.8     0.081                                            22 0.003                                                                            0.95                                                                             0.71                                                                             0.020                                                                            0.006                                                                            2.55                                                                             7.7     0.108                                                                            0.05                                          23 0.018                                                                            0.87                                                                             0.50                                                                             0.020                                                                            0.005                                                                            2.75                                                                             7.9                                                                             0.05                                                                             0.03                                                                             0.107                                            24 0.003                                                                            0.41                                                                             0.31                                                                             0.023                                                                            0.006                                                                            2.11                                                                             8.1                                                                             0.18                                                   25 0.003                                                                            0.20                                                                             1.25                                                                             0.025                                                                            0.004                                                                            2.31                                                                             7.7                                                                             0.06  0.025                                            __________________________________________________________________________

                                      TABLE 2                                     __________________________________________________________________________                                         Max.                                     Composition (Wt %)       X value     corrosion                                No.                                                                              Hf Cu                                                                              Mo Sb Ni                                                                              Ca REM                                                                              N  ×1000                                                                        Processability                                                                       depth                                    __________________________________________________________________________    INVENTION STEELS                                                              1                     0.009                                                                            1.6  ◯                                                                        ◯                            2                     0.005                                                                            2.3  ◯                                                                        ◯                            3                     0.006                                                                            1.2  ◯                                                                        ◯                            4                     0.012                                                                            3.2  ◯                                                                        ⊚                         5                     0.008                                                                            0.9  ◯                                                                        ⊚                         6  0.15               0.009                                                                            1.2  ◯                                                                        ⊚                         7     1.2             0.007                                                                            13.8 ◯                                                                        ◯                            8       1.5           0.006                                                                            1.8  ◯                                                                        ◯                            9          0.21       0.007                                                                            0.5  ◯                                                                        ◯                            10            1.8     0.006                                                                            3.8  ◯                                                                        ◯                            11              0.008 0.009                                                                            1.8  ◯                                                                        ◯                            12                 0.015                                                                            0.011                                                                            20.6 ◯                                                                        ◯                            13    1.3                                                                             1.2           0.008                                                                            1.7  ◯                                                                        ◯                            14 0.18                                                                             1.1  0.06       0.009                                                                            0.2  ◯                                                                        ◯                            15    1.0     0.9     0.008                                                                            5.0  ◯                                                                        ⊚                         16    1.1       0.010 0.009                                                                            12.4 ◯                                                                        ◯                            17    1.1          0.013                                                                            0.009                                                                            0.7  ◯                                                                        ◯                            19      1.2   0.8     0.008                                                                            1.1  ◯                                                                        ⊚                         20      1.0     0.009 0.009                                                                            29.1 ◯                                                                        ⊚                         21      0.9        0.011                                                                            0.008                                                                            5.0  ◯                                                                        ⊚                         22         0.05                                                                             0.9     0.010                                                                            19.8 ◯                                                                        ⊚                         23         0.07 0.011 0.008                                                                            17.0 ◯                                                                        ⊚                         24         0.06    0.055                                                                            0.009                                                                            12.2 ◯                                                                        ⊚                         25            1.1                                                                             0.009 0.015                                                                            1.0  ◯                                                                        ⊚                         __________________________________________________________________________     (continued from Table 1, Part1)                                               The X values in the Table are the value obtained by calculation from the      following expression.                                                         ##STR1##                                                                 

                                      TABLE 3                                     __________________________________________________________________________    Composition (Wt %)                                                            No.                                                                              C  Si Mn P  S  Al Cr Nb V  Ti Zr Ta                                        __________________________________________________________________________    INVENTION STEELS                                                              26 0.018                                                                            0.18                                                                             0.71                                                                             0.026                                                                            0.006                                                                            2.89                                                                             7.6                                                                              0.11  0.113                                           27 0.003                                                                            0.15                                                                             0.30                                                                             0.025                                                                            0.004                                                                            2.13                                                                             8.2   0.14                                               28 0.003                                                                            0.97                                                                             0.59                                                                             0.021                                                                            0.005                                                                            2.15                                                                             9.1   0.02                                                                             0.025                                           29 0.018                                                                            0.86                                                                             0.82                                                                             0.020                                                                            0.007                                                                            2.86                                                                             7.7                                                                              0,12  0.097                                           30 0.003                                                                            1.00                                                                             0.89                                                                             0.025                                                                            0.005                                                                            2.77                                                                             7.3                                                                              0,22                                                  31 0.003                                                                            0.51                                                                             0.47                                                                             0.018                                                                            0.004                                                                            2.31                                                                             5.9      0.150                                           32 0.018                                                                            0.37                                                                             0.93                                                                             0.020                                                                            0.006                                                                            2.99                                                                             8.1                                                                              0.05                                                                             0.04                                                                             0.045                                           33 0.003                                                                            0.92                                                                             0.57                                                                             0.022                                                                            0.004                                                                            2.99                                                                             8.3                                                                              0.08                                                                             0.03                                               34 0.018                                                                            0.55                                                                             0.43                                                                             0.018                                                                            0.006                                                                            2.51                                                                             7.3      0.070                                                                            0.11                                         35 0.003                                                                            0.69                                                                             0.85                                                                             0.024                                                                            0.004                                                                            2.35                                                                             8.8      0.136                                           36 0.003                                                                            0.29                                                                             0.76                                                                             0.023                                                                            0.005                                                                            2.34                                                                             6.7      0.110 0.12                                      37 0.018                                                                            0.32                                                                             0.88                                                                             0.019                                                                            0.005                                                                            2.89                                                                             7.3   0.03                                                                             0.035                                           COMPARISON STEELS                                                             38 0.005                                                                            0.48                                                                             0.39                                                                             0.019                                                                            0.004                                                                            0.05                                                                             2.3      0.110                                           39 0.008                                                                            0.17                                                                             0.85                                                                             0.019                                                                            0.005                                                                            0.29                                                                             7.3                                                                              0.03                                                  40 0.007                                                                            0.26                                                                             0.46                                                                             0.025                                                                            0.006                                                                            0.27                                                                             7.5         0.22                                         41 0.047                                                                            0.82                                                                             0.13                                                                             0.023                                                                            0.006                                                                            0.22                                                                             9.3            0.02                                      42 0.061                                                                            0.24                                                                             0.35                                                                             0.028                                                                            0.006                                                                            0.04                                                                             5.2                                                      43 0.012                                                                            0.12                                                                             0.22                                                                             0.026                                                                            0.004                                                                            0.28                                                                             9.8                                                      44 0.018                                                                            0.20                                                                             0.47                                                                             0.027                                                                            0.004                                                                            0.03                                                                             9.3                                                      45 0.015                                                                            0.33                                                                             1.04                                                                             0.019                                                                            0.003                                                                            0.04                                                                             8.6      0.010                                           46 0.045                                                                            0.76                                                                             1.04                                                                             0.027                                                                            0.005                                                                            0.03                                                                             9.2                                                      47 0.028                                                                            0.41                                                                             0.71                                                                             0.025                                                                            0.005                                                                            0.03                                                                             9.1                                                                              0.02     0.01                                         48 0.028                                                                            0.63                                                                             0.76                                                                             0.018                                                                            0.007                                                                            0.03                                                                             11.5     0.009                                           49 0.012                                                                            1.40                                                                             0.71                                                                             0.026                                                                            0.006                                                                            0.05                                                                             10.8                                                                             0.15                                                                             0.05                                                                             0.050                                           __________________________________________________________________________     (continued from Table 1, Part2)                                          

                                      TABLE 4                                     __________________________________________________________________________                                         Max.                                     Composition (Wt %)       X value     corrosion                                No.                                                                              Hf Cu                                                                              Mo Sb Wi                                                                              Ca REM                                                                              N  '10000                                                                             Processability                                                                       depth                                    __________________________________________________________________________    INVENTION STEELS                                                              26            1.3  0.062                                                                            0.008                                                                            18.8 ◯                                                                        ⊚                         27              0.008                                                                            0.082                                                                            0.008                                                                            20.7 ◯                                                                        ⊚                         28    1.0                                                                             1.2                                                                              0.08       0.010                                                                            1.2  ◯                                                                        ⊚                         29    1.1                                                                             1.1   1.1     0.013                                                                            13.5 ◯                                                                        ⊚                         30    1.0                                                                             1.3     0.008 0.010                                                                            15.9 ◯                                                                        ⊚                         31    1.1                                                                             1.2        0.005                                                                            0.013                                                                            21.6 ◯                                                                        ⊚                         32    0.3                                                                             0.8                                                                              0.05                                                                             1.2     0.016                                                                            1.2  ◯                                                                        ⊚                         33    0.8                                                                             0.7                                                                              0.09 0.009 0.017                                                                            2.8  ◯                                                                        ⊚                         34    1.2                                                                             1.5                                                                              0.05    0.066                                                                            0.013                                                                            7.4  ◯                                                                        ⊚                         35    0.8                                                                             0.7                                                                              0.03                                                                             0.8                                                                             0.003 0.009                                                                            21.1 ◯                                                                        ⊚                         36    0.9                                                                             0.6                                                                              0.04                                                                             1.0  0.072                                                                            0.010                                                                            21.7 ◯                                                                        ⊚                         37 0.15                                                                             1.1                                                                             0.9                                                                              0.08                                                                             0.5                                                                             0.09                                                                             0.088                                                                            0.010                                                                            3.8  ◯                                                                        ⊚                         COMPARISON STEELS                                                             38                    0.011                                                                            13.5 ◯                                                                        X                                        39                    0.014                                                                            -10.3                                                                              X      X                                        40                    0.014                                                                            11.2 ◯                                                                        X                                        41                    0.11                                                                             -36.5                                                                              X      X                                        42 0.02            0.032                                                                            0.006                                                                            -43.0                                                                              X      X                                        43              0.003 0.008                                                                            -12.6                                                                              X      X                                        44      1.5           0.007                                                                            -16.0                                                                              X      X                                        45         0.41       0.008                                                                            -12.5                                                                              X      X                                        46    1.1             0.009                                                                            -35.1                                                                              X      X                                        47            1.8     0.007                                                                            -19.4                                                                              X      X                                        48 0.01                                                                             0.1             0.009                                                                            -21.4                                                                              X      X                                        49    0.4             0.012                                                                            21.5 ◯                                                                        X                                        __________________________________________________________________________     (continued from Table 1, Part3)                                               The X values in the Table are the value obtanined by calculation from the     following expression.                                                         ##STR2##                                                                 

                                      TABLE 5                                     __________________________________________________________________________    Composition (Wt %)                                                            No.                                                                              C  Si Mn P  S  Al Cr                                                                              W  Nb V  Ti Zr                                         __________________________________________________________________________    INVENTION STEELS                                                              50 0.016                                                                            0.33                                                                             0.94                                                                             0.010                                                                            0.009                                                                            1.55                                                                             6.3                                                                             2.59                                                                             0.18                                                52 0.007                                                                            0.39                                                                             0.80                                                                             0.013                                                                            0.007                                                                            0.77                                                                             6.5                                                                             2.22        0.18                                       53 0.018                                                                            0.18                                                                             0.22                                                                             0.020                                                                            0.007                                                                            1.11                                                                             7.6                                                                             1.48                                                   54 0.015                                                                            0.68                                                                             1.47                                                                             0.029                                                                            0.009                                                                            1.69                                                                             6.1                                                                             0.32                                                                             0.15                                                                             0.08                                             55 0.009                                                                            0.33                                                                             0.74                                                                             0.018                                                                            0.008                                                                            2.62                                                                             7.4                                                                             1.35                                                                             0.08  0.05                                          56 0.015                                                                            0.11                                                                             0.39                                                                             0.027                                                                            0.010                                                                            1.32                                                                             6.6                                                                             2.00                                                                             0.12     0.08                                       57 0.006                                                                            0.95                                                                             1.41                                                                             0.005                                                                            0.004                                                                            2.16                                                                             7.1                                                                             2.95  0.07  0.05                                       59 0.017                                                                            0.96                                                                             1.30                                                                             0.006                                                                            0.006                                                                            1.45                                                                             5.6                                                                             2.35     0.11                                                                             0.08                                       61 0.007                                                                            0.50                                                                             0.84                                                                             0.010                                                                            0.009                                                                            2.80                                                                             6.8                                                                             1.45     0.07                                          65 0.011                                                                            0.25                                                                             0.81                                                                             0.006                                                                            0.010                                                                            2.85                                                                             7.1                                                                             0.93                                                                             0.08                                                                             0.05                                             67 0.005                                                                            0.23                                                                             0.37                                                                             0.019                                                                            0.006                                                                            1.13                                                                             9.3                                                                             0.59        0.09                                       69 0.013                                                                            0.91                                                                             0.40                                                                             0.019                                                                            0.004                                                                            1.49                                                                             8.8                                                                             0.91                                                                             0.15                                                                             0.02                                                                             0.03                                          71 0.006                                                                            0.83                                                                             0.27                                                                             0.027                                                                            0.004                                                                            1.01                                                                             7.5                                                                             1.78  0.15                                             72 0.013                                                                            0.74                                                                             0.46                                                                             0.019                                                                            0.005                                                                            1.92                                                                             9.8                                                                             0.32                                                   __________________________________________________________________________

                                      TABLE 6                                     __________________________________________________________________________                                             Max.                                 Composition (Wt %)           X value     corrosion                            No.                                                                              Ta Hf Cu Mo Sb Ni                                                                              Ca REM                                                                              N  ×10000                                                                       Processability                                                                       depth                                __________________________________________________________________________    INVENTION STEELS                                                              50                        0.003                                                                            6.85 ◯                                                                        ◯                        52                        0.014                                                                            7.23 ◯                                                                        ◯                        53 0.41                   0.017                                                                            1.02 ◯                                                                        ◯                        54       1.21             0.016                                                                            12.75                                                                              ◯                                                                        ◯                        55          1.78          0.005                                                                            10.08                                                                              ◯                                                                        ◯                        56             0.31       0.017                                                                            1.48 ◯                                                                        ◯                        57       1.20                                                                             1.60          0.015                                                                            6.55 ◯                                                                        ◯                        59       0.20       0.06  0.019                                                                            9.92 ◯                                                                        ◯                        61    0.17  0.49                                                                             0.44       0.010                                                                            13.71                                                                              ◯                                                                        ◯                        65 0.07        0.13    0.035                                                                            0.013                                                                            7.73 ◯                                                                        ◯                        67 0.07                                                                             0.02                                                                             2.19                                                                             0.24       0.011                                                                            0.004                                                                            8.94 ◯                                                                        ◯                        69    0.05                                                                             0.50                                                                             0.09                                                                             0.22    0.081                                                                            0.016                                                                            10.74                                                                              ◯                                                                        ◯                        71                        0.004                                                                            22.93                                                                              ◯                                                                        ⊚                     72    0.38                0.020                                                                            1.40 ◯                                                                        ⊚                     __________________________________________________________________________     (continued from Table 5, Part1)                                               The X values in the Table are the value obtained by calculation from the      following expression.                                                         ##STR3##                                                                 

                                      TABLE 7                                     __________________________________________________________________________    Composition (Wt %)                                                            No.                                                                              C  Si Mn P  S  Al Cr W  Nb V  Ti Zr                                        __________________________________________________________________________    INVENTION STEELS                                                              75 0.020                                                                            0.41                                                                             0.17                                                                             0.029                                                                            0.005                                                                            2.78                                                                             8.1                                                                              1.07  0.05                                                                             0.08                                         77 0.005                                                                            0.69                                                                             1.35                                                                             0.030                                                                            0.010                                                                            1.35                                                                             7.5                                                                              1.13                                                  78 0.004                                                                            0.06                                                                             1.46                                                                             0.030                                                                            0.004                                                                            1.92                                                                             8.3                                                                              0.90                                                                             0.04                                                                             0.03  0.08                                      79 0.006                                                                            1.10                                                                             1.42                                                                             0.017                                                                            0.009                                                                            2.04                                                                             7.4                                                                              0.64                                                                             0.22                                                                             0.09                                            80 0.016                                                                            0.17                                                                             1.45                                                                             0.010                                                                            0.008                                                                            2.13                                                                             6.6                                                                              2.90  0.12                                                                             0.07                                                                             0.02                                      81 0.013                                                                            0.12                                                                             0.74                                                                             0.024                                                                            0.009                                                                            1.68                                                                             7.8                                                                              0.20  0.05                                                                             0.06                                         82 0.011                                                                            0.14                                                                             0.84                                                                             0.027                                                                            0.007                                                                            1.91                                                                             8.3                                                                              2.43     0.04                                                                             0.05                                      83 0.013                                                                            0.67                                                                             0.88                                                                             0.023                                                                            0.003                                                                            2.78                                                                             7.7                                                                              0.53     0.08                                                                             0.07                                      84 0.008                                                                            0.36                                                                             1.14                                                                             0.023                                                                            0.010                                                                            1.73                                                                             8.3                                                                              2.38                                                                             0.11                                                                             0.03                                                                             0.03                                         85 0.006                                                                            0.33                                                                             0.48                                                                             0.012                                                                            0.006                                                                            1.78                                                                             9.4                                                                              1.51                                                                             0.04                                                                             0.03                                                                             0.05                                                                             0.05                                      86 0.016                                                                            0.85                                                                             0.57                                                                             0.010                                                                            0.010                                                                            1.86                                                                             7.3                                                                              0.44                                                                             0.07                                                                             0.03                                                                             0.07                                                                             0.06                                      COMPARISON STEELS                                                             87 0.007                                                                            0.45                                                                             1.39                                                                             0.015                                                                            0.006                                                                            0.05                                                                             2.3         0.11                                         88 0.004                                                                            0.99                                                                             0.36                                                                             0.025                                                                            0.009                                                                            0.17                                                                             7.3   0.12                                               89 0.005                                                                            0.74                                                                             1.31                                                                             0.005                                                                            0.008                                                                            0.07                                                                             7.5            0.16                                      90 0.019                                                                            0.63                                                                             1.43                                                                             0.026                                                                            0.009                                                                            0.08                                                                             9.3                                                      91 0.009                                                                            0.77                                                                             0.55                                                                             0.010                                                                            0.009                                                                            0.04                                                                             5.2                                                      92 0.007                                                                            0.48                                                                             0.59                                                                             0.011                                                                            0.008                                                                            0.11                                                                             9.8                                                      93 0.014                                                                            0.60                                                                             1.30                                                                             0.023                                                                            0.004                                                                            0.03                                                                             9.3                                                      94 0.019                                                                            0.15                                                                             0.16                                                                             0.013                                                                            0.005                                                                            0.04                                                                             8.6         0.01                                         95 0.014                                                                            0.96                                                                             1.38                                                                             0.027                                                                            0.004                                                                            0.03                                                                             9.2                                                      96 0.014                                                                            0.96                                                                             1.21                                                                             0.021                                                                            0.007                                                                            0.03                                                                             9.1   0.02     0.01                                      97 0.008                                                                            1.04                                                                             0.89                                                                             0.026                                                                            0.005                                                                            0.03                                                                             11.5        0.15                                         98 0.013                                                                            0.49                                                                             0.40                                                                             0.015                                                                            0.004                                                                            0.11                                                                             12.1                                                                             0.15                                                                             0.15                                                                             0.05                                            __________________________________________________________________________     (continued from Table 5, Part2)                                          

                                      TABLE 8                                     __________________________________________________________________________                                              Max.                                Composition (Wt %)            X value     corrosion                           No.                                                                              Ta Hf Cu Mo Sb Ni Ca REM                                                                              N  ×10000                                                                       Processability                                                                       depth                               __________________________________________________________________________    INVENTION STEELS                                                              75                      0.010                                                                            0.014                                                                            5.24 ◯                                                                        ⊚                    77 0.15                                                                             0.22  0.47        0.009                                                                            0.014                                                                            9.61 ◯                                                                        ⊚                    78             0.08  0.019 0.007                                                                            12.36                                                                              ◯                                                                        ⊚                    79    0.07        1.75                                                                             0.023 0.020                                                                            29.62                                                                              ◯                                                                        ⊚                    80                1.22  0.065                                                                            0.019                                                                            18.55                                                                              ◯                                                                        ⊚                    81    0.07                                                                             1.72                                                                             0.32                                                                             0.14        0.011                                                                            11.16                                                                              ◯                                                                        ⊚                    82 0.06  1.37                                                                             1.28  1.79     0.008                                                                            5.13 ◯                                                                        ⊚                    83    0.09                                                                             0.55                                                                             0.47     0.003 0.010                                                                            14.57                                                                              ◯                                                                        ⊚                    84 0.07  2.97                                                                             1.35                                                                             0.21  0.016 0.013                                                                            15.12                                                                              ◯                                                                        ⊚                    85 0.07  1.44                                                                             0.20                                                                             0.39                                                                             1.63                                                                             0.010 0.009                                                                            20.81                                                                              ◯                                                                        ⊚                    86    0.08                                                                             1.21                                                                             1.12                                                                             0.18                                                                             0.32  0.039                                                                            0.016                                                                            19.21                                                                              ◯                                                                        ⊚                    COMPARISON STEELS                                                             87                         0.012                                                                            11.45                                                                              ◯                                                                        X                                   88                         0.014                                                                            2.02 ◯                                                                        X                                   89                         0.019                                                                            3.50 ◯                                                                        X                                   90 0.02                    0.014                                                                            -19.69                                                                             X      X                                   91    0.02              0.032                                                                            0.010                                                                            -10.34                                                                             X      X                                   92                   0.003 0.020                                                                            -15.79                                                                             X      X                                   93          1.50           0.017                                                                            -19.59                                                                             X      X                                   94             0.41        0.008                                                                            -15.18                                                                             X      X                                   95       1.10              0.017                                                                            -19.09                                                                             X      X                                   96                1.80     0.05                                                                             -9.23                                                                              X      X                                   97    0.01                                                                             0.05              0.06                                                                             22.67                                                                              ◯                                                                        X                                   98       0.40              0.018                                                                            6.55 ◯                                                                        X                                   __________________________________________________________________________     (continued from TABE 5, Part3)                                                The X values in the Table are the valued obtained by calculation from the     following exression.                                                          ##STR4##                                                                 

We claim:
 1. A steel excellent in corrosion resistance andprocessability which, in weight percent,consistsof not less than 0.01%and less than 1.2% of Si, 0.1-1.5% of Mn, 5.5-9.9% of Cr and 1.0-3.0% ofAl, has reduced contents ofC of not more than 0.02%, P of not more than0.03%, S of not more than 0.01% and N of not more than 0.02%, furtherconsists ofone or more elements selected from among Nb, V, Ti, Zr, Taand Hf in a total amount of 0.01-0.5%, and satisfies the relationship

    (Nb/93)+(V/51)+(Ti/48)+(Zr/91)+(Ta/181)+(Hf/179)-0.8×[(C/12)+(N/14)].gtoreq.0,

the remainder being Fe and unavoidable impurities.
 2. A steel excellentin corrosion resistance and processability which, in weightpercent,consists ofnot less than 0.01% and less than 1.2% of Si,0.1-1.5% of Mn, 5.5-9.9% of Cr and 1.0-3.0% of Al, has reduced contentsofC of not more than 0.02%, P of not more than 0.03%, S of not more than0.01% and N of not more than 0.02%, further consists ofone or moreelements selected from among Nb, V, Ti, Zr, Ta and Hf in a total amountof 0.01-0.5%, and satisfies the relationship

    (Nb/93)+(V/51)+(Ti/48)+(Zr/91)+(Ta/181)+(Hf/179)-0.8×[(C/12)+(N/14)].gtoreq.0, and

as addition components further consists of, in weight percent, one ormore of0.05-3.0% of Cu, 0.05-2.0% of Mo, 0.01-0.5% of Sb, 0.01-2.0% ofNi, and 0.05-3.0% W, the remainder being Fe and unavoidable impurities.3. A steel excellent in corrosion resistance and processability which,in weight percent,consists ofnot less than 0.01% and less than 1.2% ofSi, 0.1-1.5% of Mn, 5.5-9.9% of Cr and 1.0-3.0% of Al, has reducedcontents ofC of not more than 0.02%, P of not more than 0.03%, S of notmore than 0.01% and N of not more than 0.02%, further consists ofone ormore elements selected from among Nb, V, Ti, Zr, Ta and Hf in a totalamount of 0.01-0.5%, satisfies the relationship

    (Nb/93)+(V/51)+(Ti/48)+(Zr/91)+(Ta/181)+(Hf/179)-0.8×[(C/12)+(N/14)].gtoreq.0, and

as addition components further consists of, in weight percent, one ormore of0.001-0.1% of rare earth elements, and 0.0005-0.03% of Ca, theremainder being Fe and unavoidable impurities.
 4. A steel excellent incorrosion resistance and processability which, in weightpercent,consists ofnot less than 0.01% and less than 1.2% of Si,0.1-1.5% of Mn, 5.5-9.9% of Cr and 1.0-3.0% of Al, has reduced contentsofC of not more than 0.02%, P of not more than 0.03%, S of not more than0.01% and N of not more than 0.02%, further consists ofone or moreelements selected from among Nb, V, Ti, Zr, Ta and Hf in a total amountof 0.01-0.5% satisfies the relationship

    (Nb/93)+(V/51)+(Ti/48)+(Zr/91)+(Ta/181)+(Hf/179)-0.8×[(C/12)+(N/14)].gtoreq.0,

as addition components further consists of, in weight percent, one ormore of0.05-3.0% of Cu, 0.05-2.0% of Mo, 0.01-0.5% of Sb, 0.01-2.0% ofNi, and 0.05-3.0% W, and as further addition components further consistsof, in weight percent, one or more of0.001-0.1% of rare earth elements,and 0.0005-0.03% of Ca, the remainder being Fe and unavoidableimpurities.
 5. An exhaust system for an internal combustion engine, saidexhaust system comprising a steel according to claim
 1. 6. An exhaustsystem for an internal combustion engine, said exhaust system comprisinga steel according to claim
 2. 7. An exhaust system for an internalcombustion engine, said exhaust system comprising a steel according toclaim
 3. 8. An exhaust system for an internal combustion engine, saidexhaust system comprising a steel according to claim 4.